The disclosure of Japanese Patent Application No. 2002-119352 filed on Apr. 22, 2002, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a valve condition control system and a control method thereof for controlling intake and exhaust valves of an internal combustion engine. In particular, the invention relates to such control system and method that can advantageously be used with an internal combustion engine including variable valve drive mechanisms respectively provided to intake and exhaust valves and thus capable of changing the operation condition of each valve independently.
2. Description of Related Art
There is known a variable valve drive mechanism for changing conditions of intake and exhaust valves, such as valve timing (i.e., timing of opening/closing valves) and valve lift, in order to achieve increased engine output, reduced exhaust emissions, and so on. Especially, in the case of internal combustion engines to be mounted on vehicles, for example, a so-called VVT (Variable Valve Timing) mechanism capable of changing valve timings by changing the rotational phase of each camshaft relative to the crank shaft is widely used as the variable valve drive mechanism.
Besides, an internal combustion engine equipped with a so-called dual VVT system, as disclosed in Japanese Laid-open Patent Publication No. 11-218035, has been put into practice. This engine includes VVT mechanisms that are constructed on intake side camshaft and exhaust side camshaft respectively, and changes the valve timing of each intake or exhaust valve independently.
Conventionally, in an internal combustion engine provided with a valve condition control system incorporating such a dual VVT system, a valve timing appropriate for the engine operation state is separately calculated for the intake valve and the exhaust valve each, and the valve timing calculated is used as a target valve timing of each VVT mechanism. Then, the operation of each VVT mechanism is controlled such that its actual valve timing matches the target valve timing.
According to the valve condition control system including the dual VVT system constructed as described above, the valve timing of each intake or exhaust valve is set, through the aforementioned control, to a certain timing appropriate for the engine operation state. With such a conventional valve condition control system, however, failures or problems, as described in the following, may occur when valve timings are changed by the VVT mechanisms, so some improvements are required.
By the way, one object of valve condition control systems for an internal combustion engine is to adjust a xe2x80x9cvalve overlapping amountxe2x80x9d between intake and exhaust valves. More specifically, by optimizing the valve overlapping amount according to the engine operation state, the internal EGR (Exhaust Gas Recirculation) amount in each cylinder is changed as required for reducing exhaust emissions.
In the conventional valve condition control system described above, such an optimization of valve overlapping amount is eventually accomplished after the target valve timing of each VVT mechanism is set to a timing required for achieving a most appropriate valve overlapping amount. However, after the target valve timing of each VVT mechanism is set, the most appropriate valve overlapping amount is not achieved until both the VVT mechanisms have been driven to the respective target timings, so it is difficult to assure a sufficient response speed in adjusting the valve overlapping amount. Therefore, especially under an engine operation state where the setting of valve overlapping amount largely influences exhaust emissions, and the like, problems, such as an increase in the exhaust emissions, may occur during such a time lag in bringing the actual valve timing to the target valve timing.
Furthermore, if the response speed of each VVT mechanism is different, problems, as described below, will further be caused. Even if the VVT mechanisms at the intake and exhaust sides have completely the same construction, the response speed tends to be different from each other owing to a different length of a passage for supplying oil pressures, and so on. Hereinafter, problems originating from such a difference in the response speed will be explained with reference to FIG. 7.
FIG. 7 represents a certain state of a valve timing control executed by the above-described conventional valve condition control system when controlling both intake and exhaust valves. This example shows the case where the response speed of the intake-side VVT mechanism is lower than that of the exhaust-side VVT mechanism. In the control state shown in FIG. 7, the valve timing of each VVT mechanism is to be changed towards the delay side while reducing the valve overlapping amount from a time point T.
First of all, at the time point T, target valve timings for the respective valves are calculated such that their valve timings change as mentioned above. Then, the valve timing of each valve begins to change to the delay side. At this time, however, the valve timing of the exhaust valve changes at a higher speed than the intake valve because of the difference in the response speed between the VVT mechanisms. Thus, the valve overlapping amount becomes in time larger and larger than before changing the valve timings.
When the valve timing of the exhaust valve reaches and stops at its target valve timing at the time point T1, the valve overlapping amount then starts to gradually reduce as the valve timing of the intake valve changes towards the delay side. Then, when the valve timing of the intake valve reaches and stops at its target valve timing at the time point T2, the valve-overlapping mount finally matches an amount required according to the engine operation state.
When the valve timings are changed in such a manner, it may cause such an unnecessary increase in the valve overlapping amount while changing the valve timings, and the valve overlapping amount may become temporarily larger than before changing the valve timings, although a required valve overlapping amount is eventually attained as described above. This contradicts the fact that the valve timings are changed so that the valve overlapping amount ultimately reduces. Upon such an unnecessary increase in the valve overlapping amount, the internal EGR amount in each cylinder becomes excessively large, which may increase hydrocarbons to be ejected from the internal combustion engine. Moreover, when the internal EGR amount becomes excessively large, the combustion temperature inappropriately lowers, or too much intake air passes through the combustion chamber from an intake passage to an exhaust passage, which may cause the engine combustion to become unstable, and which may even cause an engine stall in the worst case.
Conversely, if the response speed of each VVT mechanism is different and the valve timing of each valve is changed so as to increase the valve overlapping amount, it may happen that the valve overlapping amount unnecessarily reduces while changing the valve timings, and the valve overlapping amount thereby becomes smaller than before changing the valve timings. In this case, the internal ERG amount within the cylinder unnecessarily reduces, which may cause an inappropriate increase in the combustion temperature, an increase in NOx emissions, and so on.
Consequently, with such a conventional valve condition control system which merely uses a valve timing appropriate for the engine operation state as a target valve timing, the valve overlapping amount can not be always adjusted in an appropriate manner during the valve condition control. In the meantime, in the case of a valve condition control system including variable valve drive mechanism of other type than VVT mechanisms, and the like, for changing valve timings by changing relative phases of a camshaft, if that valve condition control system includes variable valve drive mechanisms constructed on both the intake and exhaust sides and is arranged to change the valve condition of each intake or exhaust valve independently, it may be subject to substantially the same problems as those described above.
In view of the above situation, the invention has been made to provide valve condition control system and method for maintaining an appropriate operation state of an internal combustion engine including variable valve drive mechanisms constructed on both the intake and exhaust sides while changing valve conditions.
To achieve the above object, a valve condition control system for an internal combustion engine is provided, which includes a first variable valve drive mechanism for changing a valve condition of one of an intake valve and an exhaust valve of the internal combustion engine; a second variable valve drive mechanism for changing a valve condition of the other of the intake valve and the exhaust valve; and a controller. The controller calculates the control amount of the first variable valve drive mechanism required for obtaining an appropriate valve condition of the one of the intake and exhaust valves for the engine operation state as a target control amount of the first variable valve drive mechanism; calculates a target valve overlapping amount between the intake and exhaust valves in accordance with the engine operation state; calculates a target control amount of the second variable valve drive mechanism on the basis of the actual control amount of the first variable valve drive mechanism and the target valve overlapping amount; and controls the operation of each variable valve drive mechanism such that its actual control amount matches the target control amount.
Also, one exemplary embodiment of the invention relates to a control method of a valve condition control system for an internal combustion engine including a first variable valve drive mechanism for changing a valve condition of one of an intake valve and an exhaust valve of the internal combustion engine and a second variable valve drive mechanism for changing a valve condition of the other of the intake valve and the exhaust valve. The valve condition control system is adapted to calculate a target control amount of each variable valve drive mechanism and to control the operation of each variable valve drive mechanism such that its actual control amount matches the target control amount. This control method includes the steps of: calculating a control amount of the first variable valve drive mechanism required for obtaining an appropriate valve condition of the one of the intake and exhaust valves for an engine operation state as a target control amount of the first variable valve drive mechanism;
calculating a target valve overlapping amount between the intake and exhaust valves in accordance with the engine operation state; and
calculating the target control amount of the second variable valve drive mechanism on the basis of the actual control amount of the first variable valve drive mechanism and the target valve overlapping amount.
According to these control system and method, the target control amount of the second variable valve drive mechanism is calculated based on the actual control amount of the first variable valve drive mechanism and the target valve overlapping amount. Thus, the operation of the second variable valve drive mechanism is controlled so as to achieve the target valve overlapping amount. Therefore, it is possible to achieve a required valve overlapping amount before the changing of the valve condition is finished for both the intake and exhaust valves, which permits a faster adjustment of the valve overlapping amount.
With the above arrangement, also, the valve overlapping amount is always changed such that it approaches the target valve overlapping amount while changing valve conditions. Therefore, an unnecessary increase or reduction in the valve overlapping amount, which may otherwise be caused at the time of changing valve conditions, is avoided, whereby an appropriate engine operation state is maintained while changing the valve timings.
Another exemplary embodiment of the invention relates to a valve condition control system for an internal combustion engine including a first variable valve drive mechanism for changing a valve condition of one of an intake valve and an exhaust valve of the internal combustion engine; a second variable valve drive mechanism for changing a valve condition of the other of the intake valve and the exhaust valve; and a controller. The controller calculates a control amount of the first variable valve drive mechanism required for obtaining an appropriate valve condition of the one of the intake and exhaust valves for the engine operation state as a first target control amount of the first variable valve drive mechanism; calculates a target valve overlapping amount between the intake and exhaust valves in accordance with the engine operation state; calculates a first target control amount of the second variable valve drive mechanism on the basis of the actual control amount of the first variable valve drive mechanism and the target valve overlapping amount; calculates control amounts of the first and second variable valve drive mechanisms required for obtaining each appropriate valve condition of the valves for the engine operation state as second target control amounts; selects target control amounts of the first and second variable valve drive mechanisms from the first and second target control amounts of each of the first and second variable valve drive mechanisms in accordance with the engine operation state at the time of calculating target control amounts of the first and second variable valve drive mechanisms; and controls the operation of each variable valve drive mechanisms such that its actual control amount matches the target control amount calculated.
Another exemplary embodiment of the invention relates to a control method of a valve condition control system for an internal combustion engine including a first variable valve drive mechanism for changing a valve condition of one of an intake valve and an exhaust valve of the internal combustion engine and a second variable valve drive mechanism for changing a valve condition of the other of the intake valve and the exhaust valve. The valve condition control system calculates a target control amount of each variable valve drive mechanism and controls the operation of each variable valve drive mechanism such that its actual control amount matches the target control amount. This control method includes the steps of:
calculating a control amount of the first variable valve drive mechanism required for obtaining an appropriate valve condition of one of the intake and exhaust valves for the engine operation state as a first target control amount of the first variable valve drive mechanism;
calculating a target valve overlapping amount between the intake and exhaust valves in accordance with the engine operation state;
calculating a first target control amount of the second variable valve drive mechanism on the basis of the actual control amount of the first variable valve drive mechanism and the target valve overlapping amount;
calculating control amounts of the first and second variable valve drive mechanisms required for obtaining each appropriate valve condition of the valve for the engine operation state as second target control amounts; and
selecting target control amounts of the first and second variable valve drive mechanisms from the first and second target control amounts of each of the first and second variable valve drive mechanisms in accordance with the engine operation state at the time of calculating target control amounts of the first and second variable valve drive mechanisms.
By the way, influences given to the engine operation state by such valve conditions may be roughly classified into those originating from the valve overlapping amount and those literally originating from the valve conditions, such as a timing for opening/closing the intake and exhaust valves.
According to the above-described control system and method, since the first target control amount of the second variable valve drive mechanism is calculated based on the actual control amount of the first variable valve drive mechanism and the target valve overlapping amount. When the target control amount of the second variable valve drive mechanism are calculated, the valve overlapping amounts can be quickly adjusted as aforementioned. Also, the valve overlapping amount is always changed such that it approaches the target valve overlapping amount while changing the valve conditions. Therefore, an unnecessary increase or reduction in the valve overlapping amount, which may otherwise be caused at the time of changing valve conditions, is avoided, whereby an appropriate engine operation state is maintained while changing the valve conditions.
On the other hand, a control amount of the second variable valve drive mechanism required for obtaining an appropriate valve condition for the engine operation state are calculated as the second target control amounts of the second variable valve mechanism. When the second target control amounts are selected as the target control amounts of the second variable valve drive mechanism, the valve condition of the valve, to be changed by the second variable valve drive mechanism, matches an appropriate valve condition for the engine operation state. That is, with the above control system and method, it is possible to quickly bring the valve condition of each valve to a valve condition required according to the engine operation state at the time of changing the valve conditions.
Namely, by switching means for calculating the target control amount of each variable valve drive mechanism according to the engine operation state, it is possible to maintain an appropriate engine operation state while changing the valve conditions.